1. Field of the Invention
Embodiments of the invention relate to a soldering method and a method of manufacturing a semiconductor device by using the soldering method.
2. Description of the Related Art
FIG. 8 is a structural view showing the configuration of a principal part of an IGBT (Insulated Gate Bipolar Transistor) module 500. The IGBT module 500 includes a base plate 1, an insulating circuit board 8 to be solder-joined thereon, a semiconductor chip 9 (consisting of IGBT chip 9a and diode chip 9b) joined on the insulating circuit board 8, a resin case frame 13 with an external lead-out terminal 14 bonded to a base plate 1, bonding wire 11 connecting the semiconductor chip 9 to the external lead-out terminal 14 and so on, and a sealing material 15 filled in the resin case frame 13. The insulating circuit board 8 is composed of a ceramic plate 5, a copper foil 6 in the rear surface region, and a copper foil 7 in the front surface region.
In general, the base plate 1 is plated with nickel and has a nickel plating layer 2. During storage of base plate 1, however, a metal oxide film such as a nickel oxide film is frequently formed on the surface of the nickel plating layer 2. Additionally, as mentioned above, the insulating circuit board 8 comprises the copper foil 6, the ceramic plate 5, and the copper foil 7 in this order, and some of the copper foils 6 and 7 are plated with nickel or the others are not plated with nickel. Moreover, the base plate 1 is joined to the copper foil 6 of the insulating circuit board 8 with a solder 12.
FIG. 9 is a schematic assembling flow diagram of the IGBT module 500.
The base plate 1 to be plated with nickel is prepared (Step A).
Next, the nickel-plated base plate 1 is heated with reducing gas (hydrogen gas), and then a nickel oxide film (metal oxide film) on the surface of the nickel plating layer 2 is reduced to be made a nickel film, and after that, a wet cleaning is provided (Step B).
Then, the base plate 1 is joined to the copper foil 6 of the insulating circuit board 8 with solder in a reducing atmosphere (Step C).
In the above-mentioned Step C, when the base plate 1 is joined to the copper foil 6 of the insulating circuit board 8, existence of the metal oxide film on the joined surface provides poor wettability of solder 12 to generate a void 23 when solder is melting. In addition, void 23 provides one of the causes leading to the failure of quality and reliability for the IGBT module 500, and a decrease in void generation is desirable.
At present, as mentioned above, it is a general method that a metal oxide film such as a nickel oxide film is reduced and changed into nickel film to be joined with solder, in a reducing gas atmosphere (hydrogen gas is usually used as reducing gas).
Additionally, there is also a method to carry out heating base plate 1 before solder joining, and at that case, it is general to carry out it reducing gas atmosphere as mentioned above.
Further, Japanese Unexamined Patent Application Publication No. H9-36299 discloses that oxide film on the surface of nickel plating film provides poor solder wettability and soldering in a reducing gas atmosphere containing hydrogen gas provides disadvantages such as complication of equipment and an increase in cost, as described below.
In addition, Japanese Unexamined Patent Application Publication No. 2002-4082 discloses that when nickel plating film is charged with hydrogen (changed into a reducing gas atmosphere) and comes into contact with a fused solder, hydrogen in the plating film makes thermal diffusion and reduces oxygen of the oxide film coating the surface to a water molecule, and consequently soldering can be performed without using a flux.
Further, Japanese Unexamined Patent Application Publication No. H5-69122 discloses that a nickel plating film is made to contain hydrogen, and a surface oxide film is reduced by hydrogen emitted from the inside of a metal by means of heat in soldering (at 220° C.), and then good soldering is possible.